Selective degeneration of dopaminergic (DA) neurons of substantia nigra pars compacta (SNc) is a critical event in the progression of Parkinson?s disease. These neurons are vulnerable as they undergo autonomous pacemaking with an unusual reliance on Ca2+-influx via CaV1.3 channels. Unlike other pacemaking neurons that remain relatively protected during PD pathogenesis, SNc neurons have larger Ca2+-currents with blunted inactivation, thus aggravating the propensity for Ca2+-overload. Given its central role, identifying regulatory proteins that tune Ca2+-channel function in SNc neurons is critical. One attractive regulatory candidate is SH3 and cysteine rich domain (stac) protein, a muscle and neuron specific protein that has emerged as a requisite component of skeletal muscle excitation-contraction machinery and as a locus for congenital myopathies. Even so, the neuronal functions of stac remains poorly defined, although stac2- mutations have been linked to childhood-onset schizophrenia. Motivated by preliminary data showing both the presence of stac in SNc neurons, and its functional role to selectively diminish Ca2+/CaM-dependent inactivation (CDI) in heterologous systems, we here dissect the molecular functions of stac, underlying mechanisms, and potential role in tuning SNc pacemaking, utilizing a bevy of molecular tools and quantitative optical and electrophysiological approaches. This project will usher in an exciting era of discovery via three specific aims. (1) How do stac isoforms tune CaV function? We seek to develop a unified mechanistic scheme for stac regulation of CaV1.3, by incorporating effects of alternative-splicing and RNA-editing, and by rigorous quantification of stac effects on CDI, baseline channel activity (PO), and surface-membrane trafficking (Nmem). (2) What are molecular determinants and mechanisms that support and refine stac regulation of CaV1? Informed by our prior mechanistic study, we seek to elucidate the molecular underpinnings for stac-regulation of CaV1.3 PO and Nmem and whether physiological or pathophysiological processes may fine-tune stac modulation. (3) How does multiprong- modulation of CaV1 by stac tune SNc pacemaking and Ca2+ signaling? Here, we elucidate whether either downregulation or upregulation of stac2 tunes endogenous CaV1 leading to alterations in pacemaking and action potential morphology of SNc neurons. This project promises unprecedented progress in elucidating multifaceted mechanisms for stac regulation of CaV1, an emerging frontier for Ca2+-channel physiology. Furthermore, this work may shed new insights into regulatory process that sculpt Ca2+-influx via CaV1.3 into SNc neurons, an important vulnerability for PD pathogenesis and a potentially promising therapeutic target.